Carlos Slim was destined for business from boyhood. His father, Julian Slim Haddad Aglamaz, was a Lebanese immigrant who invested in real estate in downtown Mexico City after the country's 1910 revolution. Every Sunday he would give his son a 5-peso allowance, requiring him to record his purchases in a ledger. In Slim's office today, on a bookshelf lined with works from or about Buffett, Getty and Rockefeller, Slim keeps five ledger books from his childhood.

He pulls one down and opens it. "This was exactly 52 years ago," he says, turning to a page split into two columns. "Here I bought a soft drink for 70 centavos. Here I bought two tortas, two albums, two doughnuts." Today he still has a weakness for sweets; the seams of his snug dress shirt tug at a waistline that defies his nickname.

By age 26 Slim had accumulated $400,000 from investments and from his mother and had married Soumaya Domit, who would bear him six children. (She died of a kidney ailment in 1999, and Slim hasn't remarried; today all three of his sons and two of his three sons-in-law work in high jobs at companies he controls; a daughter works for his museum.) Equipped with a civil engineering degree and a keen eye for unloved assets, he began buying businesses in all manner of industries. There was no overarching strategy except to make a profit.

Four decades later Slim profits on goods and services that are fundamental to the daily life of Mexicans. His companies sell them tires for their cars and build the roads they drive on; he owns 180 Sanborns, a retail-cum-restaurant chain where people shop and dine. His bank sells them mortgage loans, another outfit makes the tiles that cover their kitchen floors and his oil rigs find the fuel that heats their homes.

Slim-controlled companies, most of them cobbled together into two main holding companies, Grupo Carso and Grupo Financiero Inbursa, account for half of the combined market cap of Mexico's benchmark stock index, the 35-stock Indice de Precios y Cotizaciones. That index has soared, rising 49% in 2006, fueling Slim's enormous gain in wealth.

In the mid-1960s Slim bought a bottling plant and created a construction company and a real estate firm. It was the start of a conglomerate he later would name Grupo Carso, fusing the first syllables of his and his wife's first names. In 1976 he spent $1 million to buy 60% of Galas de México, a small printer of labels for cigarette packs. Five years later he used the cash flow from that to buy 51% of Cigatam, a cigarette company and one of Galas' biggest customers.

In 1982 Mexico was in the midst of economic collapse amid runaway inflation, interest rates and debt defaults. Even the boldest investors were bailing out. But Slim kept buying, grabbing assets at panic prices. "Everyone was very scared and wanted to take their money away to the U.S. and Europe," says Carlos Montemayor, chief of Marcatel, a Telmex rival in long-distance service. Slim "picked from among the suffering Mexican companies. I would have done that if I had had any money at the time."

Slim spent $13 million to buy insurance company Seguros de México in 1984, a property now worth $1.5 billion, even after four spinoffs. In 1985 he bought the Mexican retail chain Sanborns for $30 million, and now that holding turns a pretax profit of half a billion dollars. The following year he put up $50 million to buy Minera Frisco, a mining company. Then came Condumex, which made auto parts and cables, among other things; and Empresas Nacobre, a mining company; and more. Together with a few other holdings these companies are part of Grupo Carso, now worth $8 billion.

Slim had managed to avoid any public notice or criticism for 25 years, but his anonymity ended when he entered the telecom sector in 1990. Mexico's President Carlos Salinas de Gortari was in a privatization push, and Slim put in the winning bid to buy 51% voting control of the state-owned phone company for $1.8 billion.

The deal instantly stirred rumors, never substantiated, that Slim somehow had an inside track with his powerful pal in the presidential palace. Slim faced only two other bidders and had lined up backing from Whitacre's telco (then known as SBC) and France Télécom. Slim topped the next-highest bid--from a group that included GTE and a Mexican brokerage firm run by Slim's first cousin--by $70 million. (The government sold most of its stake on the open market shortly after Slim closed the deal, gradually selling off the rest by 2001.)

For the first seven years Telmex held a monopoly on phone service, and it succeeded in imposing some of the highest prices anywhere in the world. (All bidders were offered a similar competition-free period.) The market opened to rivals in 1997, and Telmex tangled with competitors repeatedly over the high access fees it charged them. Since 1990 it has grown from 5 million phone lines to 18 million today, and holds 90% of the Mexican landline market. Its access charges to rivals, once at 5.8 cents per minute in revenue that competitors collected on long-distance calls, now are down to a penny a minute for traffic that starts or ends on a Telmex line.

Slim doubled down in telecom in the late 1990s, spinning off Telmex's wireless business into a new company called América Móvil. The company toppled market leader Iusacell, controlled by billionaire Ricardo Salinas Pliego, by targeting low-income callers and signing them up without requiring a credit card or bank account. It offered subsidized handsets and prepaid phone cards, and cell service grew from 9 million subscribers in 2000 to 39 million today.

The family patriarch started handing daily control of his collection of companies to his offspring in the late 1990s. His oldest son, Carlos, 39, runs Grupo Carso. Second-oldest son, Marco Antonio, 38, runs Inbursa, the financial services arm with $2.5 billion in annual sales. Third son, Patrick, 37, is the chairman of América Movíl. Son-in-law Daniel Hajj Aboumrad is América Movíl's chief executive; son-in-law Arturo Elias Ayub runs the Telmex Foundation and is Slim's spokesman; a third son-in-law, an architect, is designing Slim's new art museum.

Now Slim works on acquiring another elusive target: the love of his people. He insists this isn't important to him, but he makes it clear that he is aware of his advancing age, cognizant that his time is running out. In the past decade his foundation has handed out 150,750 college scholarships, 10 million pairs of contact lenses and 66 million bicycles to poor kids who wouldn't otherwise get to school.

He now says this falls short. In the early 1990s he was doing only 1% of what he should have been doing on the charity front, Slim says. Today he's at maybe 20%. "I agree that I'm not doing enough," he says. "Because, for me, doing enough is not a problem of money. It is an issue of how much I am or am not solving problems."

On an office wall is a work of art given to him by an aging friend with "too much time on his hands," Slim says. It shows only a metal ruler mounted on white canvas and painted yellow until the 62-inch mark, then white to 75, then green up to 100. Slim says it tracks his advancing age--and his declining mental acuity. "Until here is where my brain is good," he says, pointing to the number 72. Then he eyes the green portion from 75 to 100. "Here is where my brain is not so good." Based on Slim's own math, he has five years or so to set things right.

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